Flow regulation reduces native plant cover and facilitates exotic invasion in riparian wetlands

Flow regulation reduces native plant cover and

facilitates exotic invasion in riparian wetlands

Jane A. Catford1*, Barbara J. Downes1, Christopher J. Gippel2 and Peter A. Vesk3

1Department of Resource Management & Geography, The University of Melbourne, Melbourne, Vic. 3010, Australia;2Fluvial Systems Pty Ltd, Stockton, NSW 2295, Australia; and 3School of Botany, The University of Melbourne,

Melbourne, Vic. 3010, Australia

Summary

1. River regulation and exotic plant invasion threaten riverine ecosystems, and the two often co-

occur. By altering water regimes, flow regulation can facilitate plant invasion by providing condi-

tions that directly benefit invading species, or by reducing competition from native species unsuited

to the modified conditions. Integrating water and weedmanagement has the potential to limit ripar-

ian plant invasion andmaximize the ecological benefit of environmental flows.

2. We surveyed plant communities and modelled flood histories of 24 riparian wetlands along the

regulated River Murray, south-eastern Australia. There were no suitable control rivers, so we com-

pared modelled pre- and post-regulation hydrological data to quantify hydrological change in the

study wetlands. Regression analyses revealed relationships between hydrological modification and

cover of native non-weed, native weed and exotic weed groups and 10 individual species.

3. Exotic cover was highest and native non-weed cover lowest in wetlands that had experienced

the greatest change in hydrology – a reduction in peak flow. Native weeds did not respond to

hydrological modification indicating that exotic species’ success was not reliant on their generalist

characteristics.

4. By altering habitat filters, hydrological modification caused a decline in amphibious native non-

weed species cover and simultaneously provided drier conditions that directly favoured the exotic

species group dominated by terrestrial species. Exotic species were potentially further assisted by

human-mediated dispersal.

5. Species and functional diversity was inversely related to exotic cover. By shifting the balance

between native and exotic taxa and changing community functional composition, flow regulation

may disrupt the ecological function and ecosystem services of floodplain wetlands.

6. Synthesis and applications. Worldwide, flow regulation has led to riverine ecosystems becom-

ing more terrestrial. The success of most introduced plants relies on minimal inundation. In this

study, flood magnitude was more important than frequency, timing, or duration for wetland

flora because it reflects spatial extent and depth of flooding. Augmenting natural spring floods

with environmental flows will kill terrestrial weeds and facilitate native macrophyte growth.

Combined with strategies for managing particular amphibious weeds, we recommend flows of

117 000–147 000 ML day)1 for at least 2 days every 10 years for River Murray wetland weed

management.

Key-words: biotic resistance, community assembly, diversity, environmental filters, environ-

mental water, functional composition, habitat generalists-specialists, human impacts, propa-

gule pressure, River Murray

Introduction

Ecological theory suggests that the assembly of biological com-

munities is determined, in part, by environmental conditions

that function as a series of environmental sieves, or filters

*Correspondence author. Applied Environmental Decision Anal-

ysis CERF, School of Botany, The University of Melbourne,

Melbourne, Vic. 3010, Australia. E-mail: catfordj@unimelb.

edu.au

Journal of Applied Ecology 2011, 48, 432–442 doi: 10.1111/j.1365-2664.2010.01945.x

� 2011 The Authors. Journal of Applied Ecology � 2011 British Ecological Society

(MacArthur & Levins 1967; Diamond 1975; Weiher & Keddy

1995). If a species lacks traits required to occupy the environ-

mental conditions of a habitat, it will be filtered out. Changes

to environmental filters will alter the type of organisms that

can inhabit an ecosystem, providing opportunities for exotic

species to invade, especially if the regional native species pool

lacks sufficient diversity (Moles, Gruber & Bonser 2008). By

modifying environmental conditions through climate change,

eutrophication and river regulation, for example, human

actions can indirectly alter the structure and function of biolog-

ical communities andmay facilitate species invasion.

We hypothesize that the abundance of exotic species may

increase as a result of environmental modification in two ways.

First, if native species are ‘habitat specialists’ adapted to histor-

ical conditions, they may be unable to tolerate modified envi-

ronmental conditions (Evangelista et al. 2008). The resultant

decline in habitat-specialist native plant populations will bene-

fit invading species that are ‘habitat generalists’ able to occupy

a range of conditions, as was the case with invasion of crayfish

Orconectes neglectus chaenodactylus Williams in Spring River

drainage, USA (Larson et al. 2009). Cover of native generalists

will track exotic cover and increase as well (Fig. 1a). Secondly,

if exotic species are pre-adapted (or adapt rapidly: Henery

et al. 2010) to themodified conditions, theymay be able to out-

compete both types of native species (Fig. 1b). While abun-

dance of exotic ‘habitat specialists’ would only increase under

certain environmental conditions (Fig. 1b), we predict that

abundance of exotic ‘habitat generalists’ would increase with

any type of environmental change provided the change was to

the detriment of a portion of native species (Fig. 1a). Invasion

success will be greatest when a decline in cover of native spe-

cialists is coupled with environmental conditions that favour

exotic specialists.

In this study, we examined whether changes in hydrological

conditions caused by river regulation prompted a change in

the composition of plant communities and facilitated exotic

invasion in floodplain wetlands of the River Murray, Austra-

lia. Many riparian plant species have specific ecophysiological

requirements of hydrology (Merritt et al. 2010), so changes in

flowmagnitude, duration, timing, frequency and predictability

may alter plant community composition (Arthington et al.

2006; Stromberg et al. 2007). We tested our two hypotheses

(Fig. 1) by examining proportional cover of terrestrial and

amphibious native non-weeds, native weeds and exotic weeds.

These trends indicated whether hydrological modification

altered habitat filters, or if other factors were at play. Compar-

ing the responses of exotic species with native weeds (species

native to Australia but invasive elsewhere) helps to determine

whether exotic weeds increased primarily because they are hab-

itat generalists with general ‘weedy’ traits (vanKleunen,Weber

& Fischer 2010), or whether they have specific advantages

associated with the type of hydrological modification experi-

enced, or their close relationship with humans and distinct evo-

lutionary histories (Catford, Jansson&Nilsson 2009).

In a novel approach, we used proportional change from

modelled pre- to post-regulation conditions to indicate the

extent and direction of change from historical conditions in 24

floodplain wetlands. In this study we specifically focused on

alteration of the water regime, which is thought to be the most

important disturbance agent in freshwater ecosystems (Rau-

lings et al. 2010). We examined specific effects of eight hydro-

logical change variables on eight groups of species and 10

individual species to determine whether some aspects of hydro-

logical change are more influential than others. However,

because the flood regime is multivariate, we predict that flora

will respondmore strongly to the combined change in variables

rather than single changes. We show that differences in the

composition of exotic and native species pools, and a shift in

their proportional cover, can affect the functional diversity of

these wetland plant communities. If there is a causal link

between river regulation and exotic invasion, hydrological

management that targets particular aspects of the flow regime

could be used to control riparian exotic weed populations (Ho-

well & Benson 2000; Buckley 2008), which are particularly dif-

ficult tomanage (Wadsworth et al. 2000).

Materials and methods

STUDY REGION AND STUDY SITES

Research was conducted in 24RiverMurray wetlands betweenHume

Dam at Albury and Barmah, south-eastern Australia (see map in

Fig. S1, Supporting Information). Of four spatial scales examined in

a previous study (Reaches spanning 74 km, Clumps of three wetlands

within 2 km of each other, Wetlands, Wetland sections: Catford &

Downes 2010), variation in plant community structure mostly

occurred at the scale of individual wetlands, thus suggesting that fac-

tors critically affecting wetland plants act at this scale. The study wet-

lands were spread along a contiguous 395 km long stretch of river

channel. The upstream end of the stretch was just downstream of a

major dam (Hume Dam). Midway along the stretch a largely unregu-

lated tributary (Ovens River) entered, and a major structure (Yar-

rawongaWeir) diverted water from the RiverMurray.

Regulation of the River Murray has reduced winter and spring

flows and increased summer and autumn flows in the study region

(Maheshwari, Walker &McMahon 1995). Irrigation uses 90–98% of

Pro

porti

on o

f veg

etat

ion

cove

r

Change from historical environmental conditions

(a) (b)

Fig. 1. Two hypotheses to explain the increase in exotic plant inva-

sion under altered environmental conditions: (a) Proportional cover

of exotic weeds increases (dotted line), regardless of the direction of

environmental change, as a result of a decline in native non-weed

cover (solid line). Native weed cover (dashed line) is also predicted to

increase in this scenario. (b) Exotic weed cover increases under a

change to certain environmental conditions (e.g. shorter flood dura-

tion) and is independent of native non-weed and native weed cover,

which are largely unaffected by environmental modification.

Flow regulation drives wetland invasion 433

� 2011 The Authors. Journal of Applied Ecology � 2011 British Ecological Society, Journal of Applied Ecology, 48, 432–442

the diverted water (MDBMC 1995). We analysed flood frequency for

109 years of modelled natural and current daily flows from 1891 to

2000 for gauges at Corowa (upstream of Yarrawonga Weir) and To-

cumwal (downstream of Yarrawonga Weir; combined partial dura-

tion and annual maximum series, fitted to Generalized Pareto

distribution). Small unseasonal floods increased downstream of Yar-

rawonga, which is related to the way in which water is delivered for

irrigation. Large infrequent floods (>40 to 50 years average return

interval, ARI) were not altered by regulation. Events that were

2 years ARI in the natural series were 5 years ARI in the current ser-

ies. Events of 20 year ARI in the natural series were 32 years ARI in

the current series. The 24 study wetlands varied in degree of hydrolog-

ical modification because of variation in downstream location and

elevation on the floodplain. Not all hydrological changes were associ-

ated with position along the river, but somewere.

WETLAND FLOOD REGIME MODELL ING

Changes to wetland flood regimes from river regulation were esti-

mated using daily time-step wetland water balance models generated

in Microsoft Office Excel (2003) under pre- and post-regulation con-

ditions (Fig. 2). Both models used simulated daily river flow data

(source: RiverMurrayWater;MDBC2002). Only the 10 years imme-

diately preceding field sampling were considered, as the more distant

hydrological history would have hadmuch less influence on determin-

ing current vegetation patterns of short-lived, herbaceous species.

Over the 10-year modelling period (1 February 1996–31 January

2006), the simulated post-regulation river water level data had a med-

ian divergence of 10% (0Æ22 m) from stage heights measured at the

river. For the water balance model, the starting wetland water level

was set as the mean wetland depth, and the model commenced on 1

February 1991 to allow 5 years for model convergence.

The water balance of a floodplain wetland is the net result of sev-

eral gains and losses, calculated over a selected time-step (Poiani &

Johnson 1993). The water balance models were based on river water

levels relative to wetland commence-to-flow (CTF, the river level

when the river and wetland are hydraulically connected) thresholds,

evapotranspiration, precipitation and wetland depths. We avoided

wetlands where water was extracted or diverted, and those that

received agricultural drainage. Local runoff would have been mini-

mal because of the low relief and small size of the wetland catch-

ments. There was no data available for surface-groundwater

exchange, but the high fraction of clay in the study wetlands suggests

that groundwater recharge or discharge would have been marginal.

We assumed an unconstrained hydraulic linkage between river and

wetlands such that when the river levels exceeded wetlands’ CTF

thresholds, the wetland water levels followed river levels. See Appen-

dix S1 (Supporting Information) for details and evaluation of the

water balance models.

HYDROLOGICAL MODIF ICATION

Using the modelled daily water levels, flood depth, duration and vari-

ability, river-wetland connection events, and the timing of flooding

0

1

2

3

4

5

6

7

8

0

1

2

3

4

5

6

7

8

Feb 1996

Feb1997

Feb1998

Feb 1999

Feb 2000

Feb2001

Feb 2002

Feb2003

Feb2004

Feb 2005

Feb 2006

(a) Pre-regulation

(b) Post-regulation

Date

Wet

land

wat

er le

vel r

elat

ive

to ri

ver f

low

gau

ge (m

)

Fig. 2.Modelled daily water levels from 1

February 1996 to 1 February 2006 for an

example wetland under (a) pre-regulation

and (b) post-regulation conditions. Solid

black line, wetland water level relative to

river gauge; solid grey line; river water level

at gauge; upper dashed grey line, wetland sill

level relative to gauge; lower dashed grey line,

wetland bed level (lowest point in wetland)

relative to gauge. See Table S3 (Supporting

Information) for details.

434 J. A. Catford et al.


and drawdown were calculated for both the pre- and post-regulation

scenarios. To reduce multicollinearity (indicated by Pearson correla-

tion coefficients), eight variables that indicated changes to the five

flood parameters were selected for analyses from a possible 27 vari-

ables (Table 1). To solve the problem of zero values, the extent and

direction of hydrological change (except changes in timing) were rep-

resented by:

Hydrological change ¼ log10ðpostþ 0 � 01Þ=ðpreþ 0 � 01Þ

where post indicates post-regulation values and pre indicates pre-

regulation values. For timing, change was the minimum number of

months between pre- and post-regulation conditions represented

by a percentage. If flood timing was delayed by 3 months, this was

recorded as a 50% increase in timing, whereas floods that came

3 months early were considered a 50% decrease in timing.

FLORIST IC SURVEY

Wetlands were surveyed in the austral summer using a stratified ran-

dom approach (13 December 2005–4 February 2006). Boundaries of

wetlands were defined as the point where the elevational gradient

became flat and where hydrophytes stopped occurring. Wetlands

were divided into three strata based on elevation and water depth,

which corresponded with vegetation zones at a site (Catford & Dow-

nes 2010). We used depth-strata to maximize sampling efficiency and

reduce residual error. Vegetation was surveyed at 24 random points

in each wetland (eight points per depth-stratum). Points were gener-

ated in a Geographical Information System environment (Hawth’s

Analysis Tools: Beyer 2005) based on the floodplain DEM (MDBC

2001), and located in the field using a geographical positioning system

(Garmin XL 12). At each sample point, a 1-m2 square floating quad-

rat was blindly tossed. All taxa in the quadrat were recorded, and

their foliar cover estimated using the Braun-Blanquet scale. Braun-

Blanquet scale values were converted tomid-point averages for statis-

tical analysis (Catford&Downes 2010).

Of 157 taxa recorded, 127 were identified to species-level; 139 to

genera and 148 to family (Table S1, Supporting Information author-

ity: Botanic Gardens Trust 2007). Information on species traits was

sourced from the literature (see Table S1, Supporting Information).

At the most, cover of all unidentified plant taxa (mostly daisies,

grasses and seedlings) made up <6% of wetland vegetation cover

(mean = 1Æ8%). A few of the unidentified taxamay have been exotic,

but their low cover makes it unlikely that their exclusion affected the

results. Species were categorized into eight groups based on func-

tional response to flooding (Brock&Casanova 1997), geographic ori-

gin and history of invasion (Randall 2007; Table 1 and Table S1,

Supporting Information). We could not reliably distinguish between

invasive and non-invasive exotic species, so refer to all of them as exo-

tic weeds. Mean foliar cover of the eight groups was calculated as a

proportion of total cover for each wetland (24 quadrats ⁄wetland,

Table 1. Description of eight environmental variables and 18 floristic response variables used in analyses

Variables Abbreviation

Explanatory variables

Maximum depth at lowest elevation [m] Depth max.

Standard error of mean depth at lowest elevation [m] Depth SE

Proportion of time when lowest elevation inundated [%] Total time flooded

Median duration of a single flood at lowest elevation [days] Flood duration med.

Maximum duration of a single river-wetland connection event [days] Connection duration max.

Median length of time of a single connection river-wetland event [days] Connection duration med.

Median month when mean elevation is inundated [month]* Flood time med.

Month when mean elevation becomes dry (start of drawdown)[month]* Drawdown start

Response variables

Native species of Australia [%] All natives

Terrestrial native non-weed species that inhabit dry areas of wetlands (water table below soil surface

or soil saturated) [%]

T native non-weeds

Amphibious native non-weed species that inhabit wet-dry ecotone and require inundation some

stage in life cycle [%]†

A native non-weeds

Terrestrial native weed species (i.e. native to Australia but naturalized or invasive elsewhere in

world) [%]

T native weeds

Amphibious native weed species [%]† A native weeds

Exotic (non-native) species [%] All exotics

Terrestrial exotic (non-native) species [%] T exotic weeds

Amphibious exotic species [%] A exotic weeds

10 species‡

Native non-weeds – Alternanthera denticulataTa R. Br.; Eleocharis acutaAp R. Br.§; Juncus ingensAp N.A. Wakef.§; Pseudoraphis

spinescensAp (R. Br.) Vickery.

Native weeds – Myriophyllum crispatumAp Orchard§; Paspalum distichumAp L.

Exotic weeds – Cynodon dactylonTp (L.) Pers.; Cyperus eragrostisAp Lam.§; Hypochaeris radicataTa L.; Ludwigia peploides subsp.

montevidensisAp (Spreng.) P.H. Raven.

Units used to quantify original variables in square brackets; all variables log10-transformed (value +0Æ01 for EV, value + 0Æ1 for RV),

except *not transformed, †binary transformed, §square-root transformed. Explanatory variables calculated over 10 year period; for anal-

yses, all variables were represented by percentage change from pre- to post-regulation conditions. %, percentage of 3653 days in 10 year

period; max., maximum; SE, standard error. Response variables: species origin and weed status follow Randall (2007); terres-

trial ⁄ amphibious flood response groups follow Brock & Casanova (1997); %, proportion of total vegetation cover in wetland; ‡details

for species are in Table S2. T, terrestrial; A, amphibious; P, perennial; a, annual.



Table 1). Data from the three depth-strata were given equal weight

despite differences in their areal extent. The 10 species selected for

analysis were the most prevalent and abundant taxa overall

(Table S2, Supporting Information). Species’ absolute cover was

highly correlated with their cover as a proportion of total vegetation

cover (q > 0Æ951), so – for ease of interpretation – absolute cover wasused in the analysis.

STATIST ICAL ANALYSIS

Correlation analysis (Pearson product-moment) was performed to

exclude highly correlated variables (q > 0Æ5) from regression analy-

sis. We used ordinary least squares bivariate linear regressions to

examine relationships between floristic cover (18 variables) and

hydrological change (eight variables; Quinn &Keough 2002). Scatter

plots indicated that relationships were primarily linear (quadratic and

cubic functions were unsuitable), and polynomial analyses did not

improve explanatory power and hence were not used. Most variables

required log- or square root-transformations (Table 1). Variables

dominated by zeros were transformed into presence ⁄ absence data.

Approximately seven significant results from the 144 bivariate regres-

sions would be expected by chance alone (a = 0Æ05), although each

individual test was motivated to test our hypotheses. When interpret-

ing the results, we therefore concentrate on relationships that are

most compelling as adjudged by the strength and consistency of

effects of hydrological change on flora.

To test whether flora responded more strongly to a combination of

variables indicating hydrological change or tomodification of a single

variable, we compared results from bivariate and multiple regression

analyses. Variables for inclusion in the multiple regressions were

selected using best subsets regression analysis (HamiltonWalk proce-

dure; Minitab 15 Statistical Software). While minimizing the likeli-

hood of collinearity in additive models, best subsets regression

identified complementary explanatory variables that most parsimoni-

ously explained variation in proportional cover of exotic ⁄ nativegroups. Of themultiple best subsetmodels provided for each response

variable, one or two optimal models were selected based on adjusted

r2 andMallow’s Cp (Quinn&Keough 2002).

Using the subset of explanatory variables selected by the best subset

models, multiple linear regressions were performed to determine flo-

ristic responses to the combination of variables. Even when explana-

tory variables were centred or standardized, parameter tolerances

were too low (<0Æ1) to interpret their individual effects in full models

(Quinn & Keough 2002). We therefore report overall results of the

additive models. Additive models have higher degrees of freedom, so

may overestimate significance and adjusted r2 values compared to

models that include interaction terms. systat 12 was used for multiple

regression models (Version 12.00.09, SYSTAT Software Inc. 2007,

Chicago, IL,USA).Minitab 15 was used to perform all other analyses

(Minitab Solutions: Minitab 15 Statistical Software). Alpha was set

as 0Æ05 in all tests.

Results

ABUNDANCE OF PLANT GROUPS

Averaged across the 24 wetlands, native non-weed cover

accounted for 49% of vegetation cover, 76% of which was

made up of amphibious species. Native weed cover made up

23% of total cover, 76% of which was also amphibious. In

contrast, amphibious exotic species made up 27% of exotic

species cover, which averaged 18% of total cover across the

study wetlands. There were similar numbers of terrestrial and

amphibious native species (49 and 38 species respectively),

whereas exotics were dominated by terrestrial species (51 and 6

species respectively; Table 2). The exotic ⁄native groups were

present in all sites with the exception of terrestrial native weeds

(23 ⁄24 sites), amphibious native weeds (22 ⁄24) and amphibi-

ous exotic weeds (22 ⁄24). The 10 study species were observed

in 15 sites on average (Table S2, Supporting Information).

The two native weed species generally had higher maximum

(63%) and mean absolute cover (18%) than the native non-

weed and exotic weed species, but differences were not signifi-

cant (2-sample t-tests, results not reported).

HYDROLOGICAL MODIF ICATION

The extent of hydrological modification varied among wet-

lands (Fig. 3, Table S3, Supporting Information). Most

hydrological variables only changed in one (a negative) direc-

Table 2. Characteristics of the native non-weed, native weed and

exotic weed species pools

Characteristics

Native

non-weeds

Native

weeds

Exotic

weeds

Family richness 24 14 21

Species richness 67 20 57

Forb (%) 53Æ7 55Æ0 70Æ2Graminoid (%) 46Æ3 35Æ0 24Æ6Clonal (%) 52Æ2 60Æ0 24Æ6Terrestrial (%) 56Æ7 55Æ0 89Æ5Amphibious (%) 43Æ2 45Æ0 10Æ5Perennial (%) 83Æ6 65Æ0 38Æ6Annual & Biennial (%) 11Æ9 25Æ0 71Æ9Annual (%) 10Æ4 20Æ0 59Æ6

Drawdown start†

Total time flo

oded

Flood time med.

Flood duration max.

Connection duration max.

Connection duration med.*

Depth max.

Depth SE

100

50

0

–50

–100

Cha

nge

from

pre

-reg

ulat

ion

cond

ition

s (%

)

Fig. 3. Interquartile box plots showing percentage change in values

from pre- to post-regulation conditions of eight hydrological

variables in 24 wetlands. Calculated as: Change = 100 · ((post–

pre) ⁄ pre).*Median connection duration increased by 985% at one site: outlier

not shown in graph; †change in drawdown start is based on 11 sites.



tion after regulation, thereby limiting the ability to test our

hypotheses across the full range of possible changes, as

depicted in Fig. 1. Overall, flood duration became shorter and

wetlands were connected to the river for less time. Maximum

flood depth (magnitude) decreased, but depth variability

increased.

INFLUENCE OF HYDROLOGICAL MODIFICATION ON

PLANT ABUNDANCE

Proportional cover of amphibious native non-weeds was the

response variable most strongly related to hydrological modi-

fication (Table 3 and Table S4, Supporting Information).

Under historical conditions, its proportional cover took a

wide range of values, but as hydrological modification intensi-

fied, its cover decreased, as illustrated in its response to the

reduction in maximum flood depth (Fig. 4). Amphibious

native non-weed cover increased as wetlands were connected

to the river for less time, and as the timing of drawdown

changed. Three variables accounted for 62% of variation in

amphibious native non-weed cover. Amphibious native non-

weeds made up 51% of all native cover, and their response

probably drove the relationship between hydrological modifi-

cation and the group containing all native species (49% of

variation explained; Table S4, Supporting Information).

Change in maximum connection duration was not included in

the best subset model for amphibious native non-weed cover

even though it accounted for 20% of variation on its own.

Among individual species, native non-weed species also had

the strongest response to hydrological modification (Table 3

and Table S4, Supporting Information), followed by native

weed species. Individual exotic species only showed a weak

response.

Despite the negligible response of individual exotic spe-

cies, 40% of variation in proportional cover of terrestrial

exotic weeds (as a group) was accounted for by three

hydrological variables. Terrestrial exotic cover increased

Table 3. Significant bivariate regression models between floristic

response variables and explanatory variables (absent variables lacked

significant relationships; intercepts excluded)

Response variable Model Adjusted r2

All natives �6Æ17 Depth max. 0Æ164*†A native

non-weeds

�757 Depth max. 0Æ150*�)118 Connection duration

max.

0Æ203*

�14Æ5 Drawdown start 0Æ185*All exotics �)30Æ2 Depth max. 0Æ294**T exotic weeds �)38Æ2 Depth max. 0Æ368**A. denticulata‡ �7Æ52 Depth SE 0Æ211*E. acuta‡ �110 Depth max. 0Æ285**J. ingens‡ �)127 Depth max. 0Æ220*P. spinescens‡ �91Æ6 Depth max. 0Æ256**M. crispatum¶ �112 Depth max. 0Æ211*P. distichum¶ �9Æ84 Connection duration

max.

0Æ170*†

C. eragrostis§ �12Æ4 Total time flooded 0Æ169*

*P < 0Æ05, **P < 0Æ01; †relationship became non-significant

with removal of outliers; ‡native non-weed; §exotic weed; ¶native

weed; see Table 1 for transformations.

2·0

1·8

1·6

0·00–0·02–0·04

1·5

1·0

0·5

60

40

20

0·00–0·02–0·04

2

1

0

2

1

0

0·00–0·02–0·04

1

0

–1

All natives T native non-weeds A native non-weeds

All exotics T exotic weeds A exotic weeds

Change in maximum depth(log10-transformed)

[reduction, shallower]

Pro

porti

on o

f tot

al v

eget

atio

n co

ver

Slope = 757*

Slope = –30·2** Slope = –38·2**

Slope = 6·17*0·164 adj. R2

0·294 adj. R2 0·368 adj. R2

0·150 adj. R2

Fig. 4. Relationship between cover of exotic ⁄ native groups as a proportion of total vegetation cover and change in maximum wetland depth.

Zero on the x-axes indicates no change frompre-regulation conditions. Responses of native weeds were non-significant (not shown). All variables

were log10-transformed except amphibious native non-weeds; slope, regression coefficient; other details followTable 3.



with a reduction in maximum flood depth (Fig. 4). Its rela-

tionship to hydrological modification was only slightly

stronger than that shown by all exotic species, which reflects

that terrestrial exotic species made up 73% of exotic cover.

The proportional cover of both terrestrial native species

groups did not relate to hydrological modification (based

on P values, Table S4, Supporting Information), even

though individual terrestrial native species did (e.g. Alter-

nanthera denticulata). As a group, amphibious native weed

cover was only related to hydrological modification when

variables were combined.

Maximum depth had the strongest relationship with floristic

cover of all explanatory variables (included in 8 ⁄18 bivariate

regressions and 12 ⁄18 best subsets regressions), even though it

changed the least between pre- and post-regulation conditions

(�3% mean reduction = 0Æ14 m; Table 3, Tables S3 and S4,

Supporting Information). Becausemaximum depth only chan-

ged in one direction, we can only compare the results to the left

side of the graphs in Fig. 1. Responses to the reduction in

maximum flood depth (i.e. magnitude) followed both

Fig. 1a,b: proportional cover of terrestrial exotic weeds

increased, amphibious native non-weed cover decreased, and

native weed cover did not vary as maximum depth became

shallower after regulation (Fig. 4). Like the amphibious native

non-weed group, cover of Eleocharis acuta, Pseudoraphis

spinescens and Myriophyllum crispatum decreased with a

reduction in maximum depth, but Juncus ingens increased

(Table 3).

The multiple regression models revealed that combined

changes in hydrology accounted for greater variation in floris-

tic cover than changes in isolation, as predicted. However,

there were no specific combinations of hydrological variables

that were consistently influential: the floristic variables were

related to a variety of hydrological variable combinations

(Table S4, Supporting Information).

IMPL ICATIONS OF HIGHER PROPORTIONAL COVER OF

EXOTIC SPECIES

The exotic species pool was dominated by species that are ter-

restrial, have a forb growth form, reproduce by seeds, and are

annual or biennial (Table 2). In contrast, the native species

groups were dominated by perennial species, but had similar

proportions of terrestrial and amphibious species, forbs and

graminoids, and species that have the capacity to reproduce

vegetatively and those that do not. Consistent with the species

pool trends, proportional cover of exotic weeds was negatively

correlated to several measures of wetland vegetation diversity

while native cover was positively correlated with them (Fig. 5).

Notably, wetlands with a higher proportion of native non-

weed cover had higher total species diversity (i.e. all species,

not just native species), whereas wetlands with higher propor-

tional cover of exotic species had lower species evenness

(Fig. 5). The diversity of plant species’ heights, time of flower-

ing, and months in flower were not strongly correlated with

any of the native ⁄ exotic groups.

Discussion

Using a novel comparison of modelled pre- and post-regula-

tion hydrology, we have shown that flow regulation affected

plant community composition and diversity in 24 River Mur-

ray wetlands. As predicted, exotic cover was highest and native

non-weed cover lowest in wetlands that had experienced the

greatest change in hydrology, namely a reduction in the largest

flood event in the 10-year period modelled. The trends were

driven by a decline in amphibious native non-weed cover and

an increase in terrestrial exotic weed cover. By reducing flows

to floodplain ecosystems, river regulation may prompt such a

shift towards amore terrestrial flora (Deiller,Walter &Tremo-

lieres 2001). However, in line with our ideas of habitat special-

ists and generalists, the decline in native non-weed cover with

hydrological modification, increase in exotic weed cover but a

lack of response from the native weed groups suggests that

flow regulation altered the environmental filters of these wet-

lands in more ways than just causing ‘terrestrialization’. Not-

withstanding the unidirectional change in hydrology, the

results appear to support both Hypothesis 1 and 2: by altering

habitat filters, hydrological modification caused a reduction in

native non-weed species cover and simultaneously provided

drier conditions that directly favoured the exotic species group

dominated by terrestrial species. Exotic species were poten-

tially further assisted by human-mediated dispersal.

WHY WAS THE TERRESTRIAL EXOTIC SPECIES GROUP

SO SUCCESSFUL?

Neither ‘specialist’ nor ‘generalist’ terrestrial native species

cover increased as flood magnitude and depth reduced post-

regulation.While the response of specialist native non-weeds is

consistent with Hypothesis 1, the distinct responses of native

weed and exotic weed cover implies that the success of the

exotic weeds was not reliant on their ‘weedy’, generalist charac-

teristics. Rather, the increase in exotic cover may reflect that

0·60·30·0–0·3–0·6

Species richness

Species diversity

Species evenness

Family diversity

Plant height diversity

Flowering phenology diversity

Months-in-flower diversity

Growth form diversity

Pearson correlation coefficient

Fig. 5. Correlations between measures of 24 wetlands’ floristic diver-

sity and wetlands’ proportional cover of native non-weeds (black

squares), native weeds (grey circles) and exotic weeds (crossed circles).

Diversity measures were calculated using the Shannon Weiner index;

flowering phenology refers to the start of flowering; growth form

includes trees and shrubs, graminoids, forbs.



the majority of exotic species (but not the most prevalent or

abundant, i.e. Hypochaeris radicata and Cynodon dactylon)

were specifically adapted to the new conditions – invoking

Hypothesis 2 (Fig. 1b).

Any changes that made wetlands drier would have advan-

taged the exotic species group as almost 90%of its species were

terrestrial. The lack of functional diversity and dominance of

terrestrial species in the exotic species pool probably stems

from propagule bias (i.e. an introductory bias towards species

with particular traits: Colautti, Grigorovich & MacIsaac

2006), which is not surprising given that most would have been

introduced for local agriculture and horticulture, reflecting

broader trends (Randall 2007). Unlike amphibious plants,

which are rarely introduced deliberately, terrestrial species

may be planted for landscaping, pasture improvement and

land rehabilitation (Hulme et al. 2008), which would elevate

both their propagule numbers and richness. A large species

pool is more likely to contain a competitive dominant than a

small one (Tilman, Lehman & Thomson 1997), but individual

dominant species may have a greater influence than species

richness on total abundance of invaders (Crawley et al. 1999).

Therefore, despite the small numbers of amphibious exotics,

they have the potential to constitute a large proportion of total

cover, and some are currently spreading in the region [e.g. Sag-

ittaria platyphylla (Engelm.) J.G. Sm.], if not already abundant

(e.g. Ludwigia peploides subsp. montevidensis). Hypothetically,

and in accordance with Hypothesis 1, it is possible that

amphibious exotic cover would have increased instead of ter-

restrial exotic cover if conditions had become wetter rather

than drier post-regulation. However, because most hydrologi-

cal variables only changed in one direction (which is logical

given the river supplies irrigation water), we cannot ascertain

whether exotic species are especially well-adapted to the modi-

fied conditions or whether their higher cover in particular wet-

lands relates to other factors, like human-mediated propagule

pressure or their novel evolutionary histories.

Exotic species’ distinct evolutionary histories can enable

favourable biotic interactions, like enemy release (Keane &

Crawley 2002), but the study wetlands were probably too large

for these effects to be detected. Evidence frommultiple systems

suggests that exotic plant invasion will be concentrated around

areas of human activity, probably as a response to elevated

propagule pressure (Botham et al. 2009). In the study region,

wetlands that experienced greatest reductions in flood magni-

tude were typically close to towns (q = 0Æ487), and terrestrial

exotic cover was higher in these areas (bivariate regression

slope = )0Æ0454, P = 0Æ01, adjusted r2 = 0Æ234; native coverwas unaffected). Exotic cover was also higher near roads,

human settlements and agricultural land (J.A. Catford, unpub-

lished data). This association with human activities – and par-

ticularly towns – may partially explain why terrestrial exotic

cover increased ahead of terrestrial native cover in sites most

affected by flow regulation. Owing to the close proximity of

towns to rivers and the prevalence of exotic vegetation around

human settlements, human-mediated propagule pressure is

thought to contribute to the high levels of invasion in riparian

ecosystems (Richardson et al. 2007). Propagule pressure of

exotic species can be reduced by replacing exotic pasture or

garden plants with native varieties, disposing of green waste

appropriately (e.g. compost, garden and aquarium plants),

and maintaining sufficient buffer widths between urban or

agricultural land and riparian zones (also see Sheley, Mullin &

Fay 1995).

IMPACT OF FLOW REGULATION ON WETLANDS’

FLORIST IC DIVERSITY

Hydrological modification did not directly affect floristic

diversity of the wetlands at the time of the survey (the authors,

unpublished data). However, wetland diversity was generally

higher when native species were more abundant and lower

when exotic species made up a greater proportion of total

cover (Fig. 5). By altering the balance between native and

exotic species groups that have different functional character-

istics (Table 2), flow regulation may impair the ecological

function of these riparian wetlands over time (Kyle & Leish-

man 2009). For instance, because the majority of exotic spe-

cies have an annual life history – in contrast to the perennial-

dominated native species pool – an increase in proportional

cover of exotic species may lead to changes in resource avail-

ability and local abiotic conditions (Holmes & Rice 1996),

and impede community resilience to environmental perturba-

tions (Lavorel et al. 1997). Indeed, the capacity of a commu-

nity to maintain its ecological function is often tied to its

functional composition rather than its species diversity (May-

field et al. 2010).

EFFECTS OF HYDROLOGICAL MODIF ICATION ON

WETLAND FLORA AND MANAGEMENT IMPLICATIONS

Corresponding with findings elsewhere (Deiller, Walter &

Tremolieres 2001; Elderd 2003), a reduction in flooding fol-

lowing flow regulation has altered the composition of River

Murray floodplain vegetation. We found that terrestrial exo-

tic species were abundant in areas formerly unsuitable for

terrestrial species (other than for short periods) thanks to a

reduction in the magnitude of peak flows (8Æ5–18 year ARI

under pre-regulation conditions, which occurred in October

1996). Change in maximum flood depth was the most influ-

ential hydrological variable, and was more strongly related

to exotic cover than proximity to human activities or other

factors associated with riparian invasion (e.g. grazing inten-

sity, soil nutrient levels, habitat heterogeneity: Catford 2008).

Although the proportional decrease in maximum depth was

minor compared to changes in other hydrological variables,

depth can control wetland plant establishment and survival

(Budelsky & Galatowitsch 2000) and average differences of

0Æ14 m in water depth (i.e. mean decrease on maximum

depth) are large for individual plants. More importantly

though, the reduction in flood magnitude effectively meant

that there was a reduction in wetland area that was inun-

dated: a greater proportion remained dry. This drier environ-

ment was suitable for terrestrial species, most of which were

exotic.



Flow regulation has led to riverine ecosystems becoming

more terrestrial throughout the world, but especially in large

floodplain rivers in temperate climates (Dynesius & Nilsson

1994). The shift towards drier conditions has given terrestrial

species an opportunity to invade floodplain areas that were

previously unsuitable. Introduced plant species are typically

good colonizers that can exploit the high levels of resource

availability characteristic of riparian zones (Rejmanek &Rich-

ardson 1996; Richardson et al. 2007). Reflecting their intro-

duction histories (e.g. for agriculture, landscaping), most

invading species are not specifically adapted to riparian zones

and cannot withstand flooding, as this study (Table S1, Sup-

porting Information) and other studies illustrate (Decamps,

Planty-Tabacchi & Tabacchi 1995; Deiller, Walter & Tremoli-

eres 2001). The two observations – that regulated rivers are

generally becoming drier, and that the success of most intro-

duced plants relies on minimal inundation – highlights the

potential formanaging terrestrial weeds with flooding.

Akin to using fire as a weed management tool (Lonsdale &

Miller 1993), inundating areas dominated by terrestrial weeds

should kill much of the standing crop and, in doing so, may

place native and exotic species on a more level playing field.

While terrestrial exotic species will no doubt establish between

floods, inundation will ensure that terrestrial exotic species do

not dominate wetlands for decades at a time, and will provide

an opportunity for native wetland plants to regenerate and dis-

perse. Exotic plant propagules may also be dispersed by flood-

waters (Howell & Benson 2000). However, a study that

examined the spatial structure of these plant communities

found no evidence that hydrochory (water-borne dispersal)

influenced the abundance of exotic flora (Catford & Downes

2010). This would in part reflect that most of the exotic species

are terrestrial and do not use hydrochory.

Although water abstraction and diversion means that most

riverine ecosystems are getting drier overall, ample opportu-

nity remains for amphibious species to invade, as invasion by

Phragmites australis in North America attests (Hudon, Ga-

gnon & Jean 2005). Species adapted to riparian zones differ in

their hydrological preferences, so if flood regime management

is used to control amphibious weeds, specific species must be

targeted: ‘one size’ does not fit all (Mortenson & Weisberg

2010). Sagittaria platyphylla arguably poses the greatest threat

to River Murray wetlands at present (DPI 2009), so imple-

menting a flood regime that disadvantages this invasive species

may be a priority. Preliminary evidence suggests that fluctuat-

ing wetland water levels and allowing wetlands to drawdown

in summer would be effective control strategies (J.A. Catford,

unpublished data). Rather than an alternative, such a strategy

would be complementary to the one that addresses terrestrial

weeds, though the spatial and temporal scales of implementa-

tionmay differ.

Water regimes are made up of numerous interdependent

parameters, and our study confirms that wetland plants are

affected by the flood regime as a whole. However, in this sec-

tion of the RiverMurray, if a management aim was to provide

conditions that favour native non-weeds over the current pool

of exotic species, environmental water should principally be

used to reinstate the magnitude of mid-range flow events,

rather than flood frequency or flood duration. To simulate

events equivalent to those observed in our 10 year study per-

iod, this would involve raising discharge above 113 000 ML

day)1 at Corowa and 160 000 ML day)1 at Tocumwal for at

least two consecutive days (rates of change in discharge should

mimic the natural hydrograph, so discharge should be elevated

for ‡6 days). Reintroducing a range of flood magnitudes will

ensure that wetlands located at different heights on the flood-

plain are inundated. This will limit the degree to which terres-

trial weeds dominate drier areas of the floodplain. We

therefore recommend that flows equivalent to 10–20 year ARI

under pre-regulation conditions are restored along this section

of the River Murray (Corowa: 117 000–131 000 ML day)1;

Tocumwal: 147 000–163 000 ML day)1).

While our findings indicate that the timing of these flows is

not crucial, we recommend that environmental water be used

to augment ‘natural’ floods that typically occur in spring. By

implementing these recommendations, other aspects of the

water regime (e.g. total time inundated) will shift in a way that

favours native species over exotic ones. Overall, our conclu-

sions support one of the aims of hydrological rehabilitation of

the River Murray – to facilitate recruitment of native wetland

plants by increasing the magnitude and duration of flood

events (MDBA 2009). However, our study shows that it is

flood magnitude, rather than duration, that is most important

for wetland flora.

Environmental flows are being used to ameliorate impacts

of flow regulation in rivers around the world (Poff et al.

1997). Given that environmental water allocations are typi-

cally limited, it is essential that release decisions are robust

and science-based. Studies, like this one, that examine the

response of biota to specific hydrological parameters may

indicate which aspect – if any – of the flow regime is most

important and what type of release will be of greatest ecologi-

cal value. Integrating findings from studies that collectively

examine biophysical and ecological responses to hydrological

modification can help guide the delivery of environmental

water (Arthington & Pusey 2003). By examining vegetation

responses to different aspects of hydrological modification, we

have shown that restoring the magnitude of peak flood events

in regulated rivers should help maintain the functional com-

position of floodplain vegetation and limit invasion by terres-

trial weeds.

Acknowledgements

Thanks to M. Stewardson, J. Gunn, J. Roberts, D. Strayer and M. Leishman

for feedback on an earlier version of the manuscript, and R. Jansson on a later

version.We also thankM. Collins for statistical advice, and R.White,M. Cald-

well, K. Ward, J. Hawkins, M. Copland, T. Alexander, A. Webb, J. Reid and

C. Logie for access and tours of sites. Field assistance was generously provided

by I. Conlan, B. Hradsky, A. Glaister, S. Catford, D. Marsh, C. Catford, G.

Tan, G. Vietz, E. Catford, J. Edge, N. Bainton, S. Connor, E. Rumpff, D. Borg

and E. Zavadil. Constructive feedback from two anonymous reviewers and the

editors enabled us to improve the manuscript. Funding was provided by

J.A.C.’s Australian Postgraduate Award, CSIRO Land and Water, the Coop-

erative Research Centre for Australian Weed Management, the Holsworth

Wildlife Research Fund, The University of Melbourne and the Applied Envi-

ronmentalDecisionAnalysis CERF.



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Received 12 July 2010; accepted 16December 2010

Handling Editor: Andy Sheppard

Supporting Information

Additional Supporting Information may be found in the online ver-

sion of this article.

Fig. S1. Mapof the 24 study wetlands.

Appendix S1. Details of the wetland water balance models including

model evaluation.

Table S1. Information about all plant taxa observed in the study wet-

lands, including their prevalence and cover.

Table S2. Classification details, foliar cover and prevalence of the 10

species analysed (themost abundant and prevalent taxa in survey).

Table S3. Descriptive statistics of eight flood regime variables under

pre- and post-regulation conditions, and percentage change in values

from pre- to post-regulation conditions.

Table S4. Results of best subsets multiple linear regressions for the

proportional cover of exotic ⁄ native groups and absolute cover of 10

species using eight hydrological change variables.

As a service to our authors and readers, this journal provides support-

ing information supplied by the authors. Such materials may be re-

organized for online delivery, but are not copy-edited or typeset.

Technical support issues arising from supporting information (other

thanmissing files) should be addressed to the authors.



Flow regulation reduces native plant cover and facilitates exotic invasion in riparian wetlands

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